Gelation acceleration

ABSTRACT

The invention provides a method made of steps of injecting into a wellbore, a composition comprising a solvent, a crosslinkable polymer, a crosslinking agent capable of crosslinking the polymer or forming a polymer, and a gelling accelerator selected from the group consisting of carbon dioxide, polylactic acid, encapsulated acid and latent acid; and allowing viscosity of the composition to increase and form a gel more quickly with the gelling accelerator than without.

FIELD OF THE INVENTION

This invention relates generally to the art of making and using oilfieldtreatment gels that viscosify more quickly. More particularly it relatesto fluids made of acrylamide polymer and/or copolymer with a gelationaccelerator and methods of using such fluids in a well from which oiland/or gas can be produced.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

Hydrocarbons (oil, condensate, and gas) are typically produced fromwells that are drilled into the formations containing them. For avariety of reasons, such as inherently low permeability of thereservoirs or damage to the formation caused by drilling and completionof the well, the flow of hydrocarbons into the well is undesirably low.In this case, the well is “stimulated,” for example using hydraulicfracturing, chemical (usually acid) stimulation, or a combination of thetwo (called acid fracturing or fracture acidizing).

Hydraulic fracturing involves injecting fluids into a formation at highpressures and rates such that the reservoir rock fails and forms afracture (or fracture network). Proppants are typically injected infracturing fluids after the pad to hold the fracture(s) open after thepressures are released. In chemical (acid) stimulation treatments, flowcapacity is improved by dissolving materials in the formation.

In hydraulic and acid fracturing, a first, viscous fluid called a “pad”is typically injected into the formation to initiate and propagate thefracture. This is followed by a second fluid that contains a proppant tokeep the fracture open after the pumping pressure is released. Granularproppant materials may include sand, ceramic beads, or other materials.In “acid” fracturing, the second fluid contains an acid or otherchemical such as a chelating agent that can dissolve part of the rock,causing irregular etching of the fracture face and removal of some ofthe mineral matter, resulting in the fracture not completely closingwhen the pumping is stopped. Occasionally, hydraulic fracturing is donewithout a highly viscosified fluid (i.e., slick water) to minimize thedamage caused by polymers or the cost of other viscosifiers.

When multiple hydrocarbon-bearing zones are stimulated by hydraulicfracturing or chemical stimulation, it is desirable to treat themultiple zones in multiple stages. In multiple zone fracturing, a firstpay zone is fractured. Then, the fracturing fluid is diverted to thenext stage to fracture the next pay zone. The process is repeated untilall pay zones are fractured. Alternatively, several pay zones may befractured at one time, if they are closely located with similarproperties. Diversion may be achieved with various techniques includingformation of a temporary plug using polymer gels.

Polymer gels have been widely used for conformance control of naturallyfissured/fractured reservoirs. For an overview of existing polymercompositions, reference is made to the U.S. Pat. Nos. 5,486,312 and5,203,834 which also list a number of patents and other sources relatedto gel-forming polymers.

In an effort to reduce the cost of the gelling system withoutsubstantially diminishing the effectiveness of the treatment, attemptsare known to at least partially substitute the polymer by a lessexpensive component. One way are foamable gel compositions as describedfor example in the U.S. Pat. Nos. 5,105,884, 5,203,834, and 5,513,705,wherein the polymer content is reduced at constant volume of thecomposition.

The typical components of a foamable gel composition are (a) a solvent,(b) a crosslinkable polymer, (c) a crosslinking agent capable ofcrosslinking the polymer or forming a polymer, (d) a surfactant toreduce the surface tension between the solvent and the gas, and (e) thefoaming gas, itself.

The use of CO₂ as foaming gas is desirable from an economic viewpoint,as this gas is used in many gas injection projects designed to generatean external fluid drive in the reservoir. Therefore an economic sourceof CO₂ would in principle be available for the gel foaming step.However, experiments with known gel systems showed that CO₂ when used asfoaming gas has a considerable impact on the stability of the gellingsystem. When CO₂ dissolves in water, it is converted to carbonic acid.It was found that known formulations for gelling systems either failedto gel in the presence of CO₂ gas or resulted in a gel with reducedlong-term stability.

The applicants found surprisingly that CO₂ as well as other compound canbe used to provide a foamable gelling composition having shorter gellingtime at low temperatures.

SUMMARY

In a first aspect, a method is disclosed. The method comprises the stepof injecting into a wellbore, a composition comprising a solvent, acrosslinkable polymer, a crosslinking agent capable of crosslinking thepolymer or forming a polymer, and a gelling accelerator selected fromthe group consisting of carbon dioxide, polylactic acid (PLA),encapsulated acid and latent acid; and allowing viscosity of thecomposition to increase and form a gel more quickly with the gellingaccelerator than without.

In a second aspect, a method of treating a subterranean formation from awellbore is disclosed. The formation comprises a rock made ofcarbonates. The method comprises the step of injecting into thewellbore, a composition comprising a solvent, a crosslinkable polymer, acrosslinking agent capable of crosslinking the polymer or forming apolymer, and a gelling accelerator, wherein the gelling accelerator issubstantially inert to the carbonate rock; contacting the compositionwith the subterranean formation, wherein the temperature is below 150 or130 degrees Celsius (° C.) at this contact; and allows viscosity of thecomposition to increase and form a gel more quickly with the gellingaccelerator than without.

In a third aspect, method of treating a subterranean formation from awellbore is disclosed. The method comprises the step of injecting into awellbore, a composition comprising a solvent, a crosslinkable polymer, acrosslinking agent capable of crosslinking the polymer or forming apolymer, a surfactant and carbon dioxide; contacts the composition withthe subterranean formation, wherein the temperature is below 130° C. atthis contact; and allowing viscosity of the composition to increase andform a gel more quickly with the carbon dioxide than without.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph comparing viscosity over time at 212° F. (100° C.) forFluid 1 (1.25% acrylamide sodium acrylate copolymer, 0.2%hexamethylenetetramine, and 0.1% phenyl acetate) at 400 psi of N₂, forFluid 2 (1.25% acrylamide sodium acrylate copolymer, 0.2%hexamethylenetetramine, and 0.1% phenyl acetate) at 400 psi of CO₂, andfor Fluid 3 (1.25% acrylamide sodium acrylate copolymer, 0.2%hexamethylenetetramine, 0.1% phenyl acetate, 2% CaCO₃, and 0.5% thefoaming agent solution) at 400 psi of CO₂, respectively.

FIG. 2 is a graph recording viscosity over time at 212° F. (100° C.) forFluid 1 (2% sodium acrylate acrylamide copolymer dispersed in mineraloil, 0.2% hexamethylenetetramine, and 0.1% phenyl acetate) at 400 psi ofCO₂.

FIG. 3 is a graph comparing viscosity over time at 225° F. (107° C.) forFluid 1 (1.25% acrylamide sodium acrylate copolymer, 0.2%hexamethylenetetramine, and 0.1% phenyl acetate) and Fluid 2 (1.25%acrylamide sodium acrylate copolymer, 0.2% hexamethylenetetramine, 0.1%phenyl acetate, and 1.2% polylactide fibers), respectively, at 400 psiof N₂.

FIG. 4 is a graph comparing viscosity over time at 250° F. (121° C.) forFluid 1 (1.25% acrylamide sodium acrylate copolymer, 0.2%hexamethylenetetramine, and 0.1% phenyl acetate) and Fluid 2 (1.25%acrylamide sodium acrylate copolymer, 0.2% hexamethylenetetramine, 0.1%phenyl acetate, and 0.6% polylactide fibers), respectively, at 400 psiof N₂.

FIG. 5 is a graph comparing viscosity over time at 225° F. (107° C.) forFluid 1 (1.25% acrylamide sodium acrylate copolymer, 0.2%hexamethylenetetramine, and 0.1% phenyl acetate) and Fluid 2 (1.25%acrylamide sodium acrylate copolymer, 0.2% hexamethylenetetramine, 0.1%phenyl acetate, 2% CaCO₃ powder, and 1.2% polylactide fibers),respectively, at 400 psi of N₂.

FIG. 6 is a graph comparing viscosity over time at 225° F. (107° C.) forFluid 1 (2% sodium acrylate acrylamide copolymer dispersed in mineraloil, 0.2% hexamethylenetetramine, 0.1% phenyl acetate, and 1.2%polylactide resin) at 400 psi of N₂.

DETAILED DESCRIPTION

At the outset, it should be noted that in the development of any actualembodiments, numerous implementation-specific decisions must be made toachieve the developer's specific goals, such as compliance with systemand business related constraints, which can vary from one implementationto another. Moreover, it will be appreciated that such a developmenteffort might be complex and time consuming but would nevertheless be aroutine undertaking for those of ordinary skill in the art having thebenefit of this disclosure.

The description and examples are presented solely for the purpose ofillustrating embodiments of the invention and should not be construed asa limitation to the scope and applicability of the invention. In thesummary of the invention and this detailed description, each numericalvalue should be read once as modified by the term “about” (unlessalready expressly so modified), and then read again as not so modifiedunless otherwise indicated in context. Also, in the summary of theinvention and this detailed description, it should be understood that aconcentration range listed or described as being useful, suitable, orthe like, is intended that any and every concentration within the range,including the end points, is to be considered as having been stated. Forexample, “a range of from 1 to 10” is to be read as indicating each andevery possible number along the continuum between about 1 and about 10.Thus, even if specific data points within the range, or even no datapoints within the range, are explicitly identified or refer to only afew specific, it is to be understood that inventors appreciate andunderstand that any and all data points within the range are to beconsidered to have been specified, and that inventors possession of theentire range and all points within the range disclosed and enabled theentire range and all points within the range.

As used herewith the term “gel” means a substance selected from thegroup consisting of (a) colloids in which the dispersed phase hascombined with the continuous phase to produce a viscous, jelly-likeproduct, (b) crosslinked polymers, and (c) mixtures thereof.

According to a first embodiment, the gel composition is a compositionmade from: a solvent, a crosslinkable polymer, a crosslinking agentcapable of crosslinking the polymer or forming a polymer, and a gellingaccelerator.

The solvent may be any liquid in which the crosslinkable polymer andcrosslinking agent can be dissolved, mixed, suspended or otherwisedispersed to facilitate gel formation. The solvent may be an aqueousliquid such as fresh water or a brine.

A crosslinked polymer is generally formed by reacting or contactingproper proportions of the crosslinkable polymer with the crosslinkingagent. However, the gel-forming composition need only contain either thecrosslinkable polymer or the crosslinking agent. When the crosslinkablepolymer or crosslinking agent is omitted from the composition, theomitted material is usually introduced into the subterranean formationas a separate slug, either before, after, or simultaneously with theintroduction of the gel-forming composition. The composition maycomprise at least the crosslinkable polymer or monomers capable ofpolymerizing to form a crosslinkable polymer (e.g. acrylamide, vinylacetate, acrylic acid, vinyl alcohol, methacrylamide, ethylene oxide,propylene oxide, AMPS (acrylamido-2-methylpropanesulfonic acid), andvinyl pyrrolidone). In another embodiment, the composition comprisesboth (a) the crosslinking agent and (b) either (i) the crosslinkablepolymer or (ii) the polymerizable monomers capable of forming acrosslinkable polymer.

Typically, the crosslinkable polymer is water soluble. Common classes ofwater soluble crosslinkable polymers include polyvinyl polymers,polymethacrylamides, cellulose ethers, polysaccharides, lignosulfonates,ammonium salts thereof, alkali metal salts thereof, as well as alkalineearth salts of lignosulfonates. Specific examples of typical watersoluble polymers are acrylamide polymers and copolymers, acrylicacid-acrylamide copolymers, acrylic acid-methacrylamide copolymers,polyacrylamides, partially hydrolyzed polyacrylamides, partiallyhydrolyzed polymethacrylamides, polyvinyl alcohol, polyvinylpyrrolidone, polyalkyleneoxides, carboxycelluloses,carboxyalkylhydroxyethyl celluloses, hydroxyethylcellulose,galactomannans (e.g., guar gum), substituted galactomannans (e.g.,hydroxypropyl guar), heteropolysaccharides obtained by the fermentationof starch-derived sugar (e.g., xanthan gum), and ammonium and alkalimetal salts thereof. Other water soluble crosslinkable polymers includehydroxypropyl guar, partially hydrolyzed polyacrylamides, xanthan gum,diutan gum, polyvinyl alcohol, and the ammonium and alkali metal saltsthereof.

The crosslinkable polymer is available in several forms such as a watersolution or broth, a gel log solution, a dried powder, and a hydrocarbonemulsion or dispersion. As is well known to those skilled in the art,different types of equipment are employed to handle these differentforms of crosslinkable polymers.

With respect to the crosslinking agents, these agents are organic andinorganic compounds well known to those skilled in the art. Exemplaryorganic crosslinking agents include, but are not limited to, aldehydes,dialdehydes, phenols, substituted phenols, hexamethylenetetramine andethers. Phenol, phenyl acetate, resorcinol, glutaraldehyde, catechol,hydroquinone, gallic acid, pyrogallol, phloroglucinol, formaldehyde, anddivinylether are some of the more typical organic crosslinking agents.Typical inorganic crosslinking agents are polyvalent metals, chelatedpolyvalent metals, and compounds capable of yielding polyvalent metals.Some of the more common inorganic crosslinking agents include chromiumsalts, aluminates, gallates, dichromates, titanium chelates, aluminumcitrate, chromium citrate, chromium acetate, and chromium propionate.

The gelling accelerator may be polylactic acid (PLA) fibers or particlesor other type of components which generally either hydrolyze orthermally decompose to form an acid downhole. As well, the gellingaccelerator may be encapsulated acid or latent acid.

According to a second embodiment, the gel composition is a compositionmade from: a solvent, a crosslinkable polymer, a crosslinking agentcapable of crosslinking the polymer, a surfactant and a gellingaccelerator embodied as a foaming gas.

Surfactants may be used to reduce the surface tension between thesolvent and the gas. The surfactants may be water-soluble and havesufficient foaming ability to enable the composition, when traversed bya gas, to foam and, upon curing, form a foamed gel. Typically, thesurfactant is used in a concentration of up to about 10, about 0.01 toabout 5, about 0.05 to about 3, or about 0.1 to about 2 weight percent.

The surfactant may be substantially any conventional anionic, cationicor nonionic surfactant. Anionic, cationic and nonionic surfactants arewell known in general and are commercially available. Exemplarysurfactants include, but are not limited to, alkyl polyethylene oxidesulfates, alkyl alkylolamine sulfates, modified ether alcohol sulfatesodium salt, sodium lauryl sulfate, perfluoroalkanoic acids and saltshaving about 3 to about 24 carbon atoms per molecule (e.g.,perfluorooctanoic acid, perfluoropropanoic acid, and perfluorononanoicacid), modified fatty alkylolamides, polyoxyethylene alkyl aryl ethers,octylphenoxyethanol, ethanolated alkyl guanidine-amine complexes,condensation of hydrogenated tallow amide and ethylene oxide, ethylenecyclomido 1-lauryl, 2-hydroxy, ethylene sodium alcoholate, methylenesodium carboxylate, alkyl arylsulfonates, sodium alkyl naphthalenesulfonate, sodium hydrocarbon sulfonates, petroleum sulfonates, sodiumlinear alkyl aryl sulfonates, alpha olefin sulfonates, condensationproduct of propylene oxide with ethylene oxide, sodium salt of sulfatedfatty alcohols, octylphenoxy polyethoxy ethanol, sorbitan monolaurate,sorbitan monopalmitate, sorbitan trioleate, polyoxyethylene sorbitantristearate, polyoxyethylene sorbitan tristearate, polyoxyethylenesorbitan monooleate, dioctyl sodium sulfosuccinate, modified phthalicglycerol alkyl resin, octylphenoxy polyethoxy ethanol, acetylphenoxypolyethoxy ethanol, dimethyl didodecenyl ammonium chloride, methyltrioctenyl ammonium iodide, sodium tridecyl ether sulfate, trimethyldecenyl ammonium chloride, and dibutyl dihexadecenyl ammonium chloride.

In one embodiment the gel composition comprises a surfactant made ofalcohol ether sulfates (AES). Alcohol ether sulfates provide a goodfoaming performance in acid brines with a broad range of ionic strengthand hardness. They allow the liquid phase of the foam to form a strongand robust gel under acid conditions.

The foaming gas is usually a noncondensable gas. Exemplarynoncondensable gases include air, oxygen, hydrogen, noble gases (helium,neon, argon, krypton, xenon, and radon), natural gas, hydrocarbon gases(e.g., methane, ethane), nitrogen, and carbon dioxide.

The amount of gas injected (when measured at the temperature andpressure conditions in the subterranean formation being treated) isgenerally about 1 to about 99 volume percent based upon the total volumeof treatment fluids injected into the subterranean formation (i.e., thesum of the volume of injected gas plus the volume of injected foamable,gel-forming composition). According to one embodiment, the amount of gasinjected is about 20 to about 98, and more preferably about 40 to about95, volume percent based upon the total volume of injected treatmentfluids.

According to a first aspect, the gel composition with the gellingaccelerator is especially suitable for downhole application in lowtemperatures below 300° F. (149° C.), or below 250° F. (121° C.), orbelow 225° F. (107° C.) or even below 200° F. (93° C.). The compositiongelation will be primarily controlled by thermal release of the activecrosslinker from crosslinking agents. This thermal reaction can be slowat lower temperatures, a gelling accelerator or gelation accelerator issuitable.

According to a second aspect, the gel composition with the gellingaccelerator is especially suitable for downhole application in lowtemperatures below 300° F. (149° C.), or below 250° F. (121° C.), orbelow 225° F. (107° C.) or even below 200° F. (93° C.) when used insubterranean formation with carbonates formation. In prior artsolutions, usually the gelation was accelerated by adding 0.1-0.4% byweight acid, e.g. acetic acid. However, when treating carbonatereservoirs, acid will be consumed by reaction with the rock and thecomposition gelation will be primarily controlled by thermal release ofthe active crosslinker from crosslinking agents. This thermal reactioncan be slow at lower temperatures, and alternative accelerators asdisclosed herewith are therefore needed for the gelation in carbonatereservoirs at relatively low temperatures.

The composition gels are compatible with other fluids or material as forexample hydrocarbons such as mineral oil, proppants or additivesnormally found in well stimulation. Current embodiments can be used invarious applications including temporary plugs formation, kill plugs, ormultiple fracturing steps for to treating subterranean formations havinga plurality of zones of differing permeabilities.

To facilitate a better understanding of some embodiments, the followingexamples of embodiments are given. In no way should the followingexamples be read to limit, or define, the scope of the embodimentsdescribed herewith.

EXAMPLES

Series of experiments were conducted to demonstrate properties ofcompositions and methods as disclosed above.

Example 1

In a first example, three fluids are shown in FIG. 1. Fluid 1 wasprepared by hydrating 1.25% acrylamide sodium acrylate copolymer inwater, followed by the addition of 0.2% hexamethylenetetramine and 0.1%phenyl acetate. Fluid 1 was used as the control sample. The viscosity at212° F. (100° C.) was measured with a Fann50-type viscometer at 400 psiof nitrogen (N₂). The fluid viscosity slowly went up over time, reaching200 cP after about 500 minutes. Fluid 2 was similarly prepared as Fluid1 with water, 1.25% acrylamide sodium acrylate copolymer, 0.2%hexamethylenetetramine, and 0.1% phenyl acetate. The difference was thatthe viscosity of Fluid 2 at 212° F. was measured with a Fann50-typeviscometer at 400 psi of carbon dioxide (CO₂). The viscosity of Fluid 2quickly went up after about 100 minutes, reaching 1000 cP at about 300minutes, suggesting that CO₂ accelerated the fluid gelation at 212° F.Fluid 3 was similarly prepared as Fluid 1 with water, 1.25% acrylamidesodium acrylate copolymer, 0.2% hexamethylenetetramine, and 0.1% phenylacetate. To simulate the carbonate formation, 2% CaCO₃ powder (FisherChemical) was mixed into Fluid 3. The foaming agent solution at 0.5% wasalso added into Fluid 3. The viscosity of Fluid 3 at 212° F. wasmeasured with a Fann50-type viscometer at 400 psi of CO₂. The behaviorof Fluid 3 was similar to that of Fluid 2, suggesting that carbonate orthe foaming agent did not have negative impact on the gelationacceleration by CO₂.

Example 2

In a second example, test was performed to see if mineral oil hadnegative impact on the gelation acceleration by CO₂. To prepare Fluid 1,instead of acrylamide sodium acrylate copolymer, sodium acrylateacrylamide copolymer dispersed in mineral oil was used at 2%, along with0.2% hexamethylenetetramine and 0.1% phenyl acetate. The viscosity ofFluid 1 at 212° F. (100° C.) was measured with a Fann50-type viscometerat 400 psi of CO₂ and shown in FIG. 2. Fluid 1 behaved qualitatively thesame as Fluid 2 in Example 1, suggesting that the mineral oil did nothave negative impact on the gelation acceleration by CO₂.

CO2 may be added to the fluids as the energizing gas, or may begenerated by the decomposition of chemicals downhole, or may begenerated by other chemical reactions downhole (e.g., acid reacts withcarbonate formation).

Example 3

In a third example, Fluid 1 was prepared by hydrating 1.25% acrylamidesodium acrylate copolymer in water, followed by the addition of 0.2%hexamethylenetetramine and 0.1% phenyl acetate. Fluid 1 was used as thecontrol sample. The viscosity at 225° F. (107° C.) was measured with aFann50-type viscometer at 400 psi of nitrogen (N₂) and shown in FIG. 3.Fluid 2 was similarly prepared as Fluid 1 with water, 1.25% acrylamidesodium acrylate copolymer, 0.2% hexamethylenetetramine, and 0.1% phenylacetate. About 1.2% polylactide fibers were then mixed into Fluid 2. Theviscosity at 225° F. was similarly measured at 400 psi of N₂ and shownin FIG. 3. The viscosity of Fluid 2 rose at a much faster rate than thatof Fluid 1 after about 400 minutes, suggesting that the polylactidefibers contributed to the gelation acceleration of the fluid at 225° F.

Example 4

Fluid 1 was prepared by hydrating 1.25% acrylamide sodium acrylatecopolymer in water, followed by the addition of 0.2%hexamethylenetetramine and 0.1% phenyl acetate. Fluid 1 was used as thecontrol fluid. The viscosity at 250° F. (121° C.) was measured with aFann50-type viscometer at 400 psi of N₂ and shown in FIG. 4. Fluid 2 wassimilarly prepared as Fluid 1 with water, 1.25% acrylamide sodiumacrylate copolymer, 0.2% hexamethylenetetramine, and 0.1% phenylacetate. About 0.6% polylactide fibers were then mixed into Fluid 2. Theviscosity at 250° F. was similarly measured at 400 psi of N₂ and shownin FIG. 2. The viscosity of Fluid 2 rose at a much faster rate than thatof Fluid 1 after about 250 minutes, suggesting that the polylactidefibers contributed to the gelation acceleration of the fluid at 250° F.

Example 5

Fluid 1 was prepared with water, 1.25% acrylamide sodium acrylatecopolymer, 0.2% hexamethylenetetramine, and 0.1% phenyl acetate. Fluid 2was prepared with water, 1.25% acrylamide sodium acrylate copolymer,0.2% hexamethylenetetramine, and 0.1% phenyl acetate. About 100 pptpolylactide fibers were then mixed into Fluid 2. To simulate thecarbonate formation, 2% CaCO₃ powder (Fisher Chemical) was also mixedinto Fluid 2. The viscosity of Fluid 1 and Fluid 2 at 225° F. (107° C.)was similarly measured at 400 psi of N₂ and shown in FIG. 5. Theviscosity of Fluid 2 rose at a much faster rate than that of Fluid 1(the control) after about 380 minutes, suggesting that the carbonate didnot affect the ability of the polylactide fibers to accelerate thegelation at 225° F.

Example 6

In this example, we tested if mineral oil had negative impact on thegelation acceleration by polylactide fibers. To prepare Fluid 1, insteadof acrylamide sodium acrylate copolymer, sodium acrylate acrylamidecopolymer dispersed in mineral oil was used at 2%, along with 0.2%hexamethylenetetramine and 0.1% phenyl acetate. About 1.2% polylactidefibers were then mixed into Fluid 1. The viscosity of Fluid 1 at 225° F.(107° C.) was measured with a Fann50-type viscometer at 400 psi of N₂and shown in FIG. 6. Fluid 1 behaved qualitatively the same as Fluid 2in Example 3 at 225° F., suggesting that the mineral oil did not havenegative impact on the gelation acceleration by polylactide fibers.

Example 7

In this example, we tested if encapsulated acid works as gellingaccelerator. Bottle tests were performed by adding 10 mL of aformulation to a crimp top chromatography vial sealed with a rubberstopper. These ampoules severely retard air intrusion into the gel andwater vapor escape from the gel. Typical screw cap vials tend to dry outover time when held at elevated temperatures. The formulations includepartially hydrolyzed polyacrylamide polymer dissolved in deionizedwater, hexamethylenetetramine, phenyl acetate, and either live orencapsulated acid. Acetic acid is used for live acid in theseexperiments while samples of encapsulated citric and fumaric acid weretested. The amount of acid was estimated from prior work and wasdecreased as the storage temperature increased. The ampoules' head spaceis briefly purged with argon to remove most of the reactive oxygen fromthe air before the ampoules are crimped. Ampoules are then placed intovarious ovens maintained at a constant temperature.

Periodically, the ampoules are removed from the oven, inverted andvisually rated, and then returned to the oven. The letter grade isrecorded according to the chart shown in the table 1 below.

TABLE 1 A No detectable gel formed: The gel appears to have the sameviscosity (fluidity) as the original polymer solution and no gel isvisually detectable. B Highly flowing gel: The gel appears to be onlyslightly more viscous (less fluid) than the original polymer solution. CFlowing gel: Most of the obviously detectable gel flows to the bottlecap upon inversion. D Moderately flowing gel: Only a small portion(about 5 to 15%) of the gel does not readily flow to the bottle cap uponinversion-- usually characterized as a “tonguing” gel (i.e., afterhanging out of jar, gel can be made to flow back into bottle by slowlyturning bottle upright). E Barely flowing gel: The gel can barely flowto the bottle cap and/or significant portion (>15%) of the gel does notflow upon inversion. F Highly deformable nonflowing gel: The gel doesnot flow to the bottle cap upon inversion. G Moderately deformablenonflowing gel: The gel flows about half way down the bottle uponinversion. H Slightly deformable nonflowing gel: The gel surface onlyslightly deforms upon inversion. I Rigid gel: There is no gel-surfacedeformation upon inversion. J Ringing rigid gel: A tuning-fork-likemechanical vibration can be felt after tapping the bottle. % XSyneresis: The amount of separated liquid is shown as a percentage ofthe original volume of fluid. This suggests instability and is to beavoided. Minor amounts of 10% or less pose no problems.

Four temperatures were used in these initial experiments and includeambient (70° F.), 150, 175 and 200° F. Three ampoules were prepared ateach temperature. The first used acetic acid, the second encapsulatedfumaric acid and the third was encapsulated citric acid. Note thatacetic acid is monoprotic, fumaric acid is diprotic and citric acid istriprotic. The encapsulating layer was a lipid that softens and melts atelevated temperature. The fumaric acid was 75% active while the citricacid was 72% active

Table 2 below shows the concentrations tested. Clearly the gelationresults are quite comparable for the different acids and concentrationsused. The ambient formulations take several days and normally wouldrequire hydrochloric acid to achieve gelation in less than eight hours.Despite the increased temperature, the 200° F. gels were slower toachieve the same state of gelation because the acid content was lower.Using slightly more acid would have achieved a faster gelation.

TABLE 2 wt % wt % wt % acetic fumaric citric T Gel rating at indicatedhours No. acid acid acid (° F.) 1 2 3 4 5 6 7 22 46 122 1 0.8 70 A A A AA A A A A A 2 1.1 70 A A A A A A A A A A 3 1.2 70 A A A A A A A A A A 40.8 150 A A A A A A A I J J 5 1.1 150 A A A A A A A I J J 6 1.2 150 A AA A A A A J J J 7 0.3 175 A A H I I I I I I I 8 0.42 175 A A H I I I I II I 9 0.44 175 A A G I I I I I I I 10 0.08 200 A A C E H I I H I I 110.11 200 A A D H I I I H I I 12 0.12 200 A A D E H I I H I I

The particular embodiments disclosed above are illustrative only, as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails herein shown, other than as described in the claims below. It istherefore evident that the particular embodiments disclosed above may bealtered or modified and all such variations are considered within thescope of the embodiments described herewith. Accordingly, the protectionsought herein is as set forth in the claims below.

1. A method comprising: a. injecting into a wellbore, a compositioncomprising a solvent, a crosslinkable polymer, a crosslinking agentcapable of crosslinking the polymer or forming a polymer, and a gellingaccelerator selected from the group consisting of carbon dioxide,polylactic acid, encapsulated acid and latent acid; b. allowingviscosity of the composition to increase and form a gel more quicklywith the gelling accelerator than without.
 2. The method of claim 1,wherein the composition further comprises a foaming agent and asurfactant.
 3. The method of claim 2, wherein the foaming agent is thegelling accelerator.
 4. The method of claim 3, wherein the gellingaccelerator is carbon dioxide.
 5. The method of claim 1, wherein theencapsulated acid or latent acid is sodium metabisulfite.
 6. The methodof claim 1, wherein the solvent is water or brine.
 7. The method ofclaim 1, wherein the crosslinkable polymer comprises acrylamide polymerand copolymer.
 8. A method of treating a subterranean formation at leastpartially made of carbonate rock comprising: a. injecting into awellbore, a composition comprising a solvent, a crosslinkable polymer, acrosslinking agent capable of crosslinking the polymer or forming apolymer, and a gelling accelerator, wherein the gelling accelerator issubstantially inert to the carbonate rock; b. contacting the compositionwith the subterranean formation, wherein the temperature is below 150degrees Celsius at this contact; c. allowing viscosity of thecomposition to increase and form a gel more quickly with the gellingaccelerator than without.
 9. The method of claim 8, wherein thecomposition further comprises a foaming agent and a surfactant.
 10. Themethod of claim 9, wherein the foaming agent is the gelling accelerator.11. The method of claim 10, wherein the gelling accelerator is carbondioxide.
 12. The method of claim 8, wherein the gelling accelerator iscarbon dioxide.
 13. The method of claim 8, wherein the gellingaccelerator is polylactic acid fiber.
 14. The method of claim 8, whereinthe gelling accelerator is a delayed acid release component.
 15. Themethod of claim 14, wherein the gelling accelerator is encapsulated acidor latent acid.
 16. The method of claim 15, wherein the encapsulatedacid or latent acid is sodium metabisulfite.
 17. The method of claim 8,wherein the solvent is water or brine.
 18. The method of claim 8,wherein the crosslinkable polymer comprises acrylamide polymer andcopolymer.
 19. The method of claim 8, wherein the temperature is below130 degrees Celsius.
 20. The method of claim 8, wherein the temperatureis below 110 degrees Celsius.
 21. The method of claim 8, wherein thetemperature is below 95 degrees Celsius.
 22. A method of treating asubterranean formation made at least partially of carbonate rockcomprising: a. injecting into a wellbore, a composition comprising asolvent, a crosslinkable polymer, a crosslinking agent capable ofcrosslinking the polymer or forming a polymer, a surfactant and carbondioxide; b. contacting the composition with the subterranean formation,wherein the temperature is below 130 degrees Celsius at this contact; c.allowing viscosity of the composition to increase and form a gel morequickly with the carbon dioxide than without.
 23. The method of claim22, wherein the solvent is water or brine.
 24. The method of claim 22,wherein the crosslinkable polymer comprises acrylamide polymer andcopolymer.
 25. The method of claim 22, wherein the temperature is below110 degrees Celsius.